High-voltage metal oxide semiconductor device and fabrication method thereof

ABSTRACT

A high-voltage metal oxide semiconductor device comprising a main body of a first conductivity type, a conductive structure, a first well of a second conductivity type, a source region of the first conductivity type, and a second well of the second conductivity type is provided. The conductive structure has a first portion and a second portion. The first portion is extended from an upper surface of the main body into the main body. The second portion is extended along the upper surface of the main body. The first well is located in the main body and below the second portion. The first well is kept away from the first portion with a predetermined distance. The source region is located in the first well. The second well is located in the main body and extends from a bottom of the first portion to a place close to a drain region.

RELATED APPLICATIONS

This application is a Divisional patent application of co-pendingapplication Ser. No. 12/285,179, filed on 30 Sep. 2008. The entiredisclosure of the prior application, Ser. No. 12/285,179, from which anoath or declaration is supplied, is considered a part of the disclosureof the accompanying Divisional application and is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high-voltage metal oxide semiconductordevice and a fabrication method thereof, and more particularly relatesto a high-voltage metal oxide semiconductor device with a vertical welland a fabrication method thereof.

2. Description of Related Art

Among various power semiconductor devices, the metal oxide semiconductorfield effect transistor, with the advantages of fast switching, lowswitching loss, and low driving loss, has been widely used forhigh-frequency power conversion. However, it is hard for the traditionalpower semiconductor device to withstand high voltage. In order toenhance withstanding voltage, on-resistance of the power semiconductormay increase disproportionately, which results in huge conduction lossand also seriously restricts the application of the power semiconductordevice.

Referring to FIGS. 1 and 1A, on-resistance of the traditionalhigh-voltage semiconductor field effect transistor (R_(DS(on))) isdominated by the resistance of the drift zone, which includes R_(ch),R_(a), and R_(epi) as shown. The voltage blocking capability of thehigh-voltage semiconductor field effect transistor is mainly decided bythe distance of the drift zone and the doping. That is, in order toincrease withstanding voltage, the epitaxial layer should be thickenedand the doping concentration should be lightened. However, the thickenedepitaxial layer and the lightened doping concentration results indisproportionate increasing of on-resistance.

The percentage of the epitaxial layer contributed to the overallon-resistance varies with the withstanding voltage. As shown, for themetal oxide semiconductor designed to withstand the voltage (V_(GD)) of30V, the epitaxial layer contributes only 29% of the totalon-resistance, whereas, for the metal oxide semiconductor designed towithstanding the voltage (V_(GD)) of 600V, the epitaxial layercontributes 96.5% of the total on-resistance.

There are two typical methods to reduce the total on-resistance of thehigh-voltage metal oxide semiconductor device. The first one is toincrease the cross-section area of the transistor so as to reduceon-resistance crossing the epitaxial layer. However, the integrationdensity must be reduced and the cost is increased. The other one is tointroduce minority carriers. However, this method not only slow down theswitching speed but also result in the existence of tail current thatincreases switching loss.

Since both the above two methods have the unsolvable drawbacks, it iseager to find out a new high-voltage metal oxide semiconductor devicewith both low on-resistance and high voltage blocking capability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a high-voltage metaloxide semiconductor device with low on-resistance for reducing powerloss and high voltage blocking capability.

A high-voltage metal oxide semiconductor device is provided in thepresent invention. The high-voltage metal oxide semiconductor devicecomprises a main body of a first conductivity type, a conductivestructure, a first well of a second conductivity type, a source regionof the first conductivity type, and a second well of the secondconductivity type. The conductive structure has a first portion and asecond portion. The first portion is extended from an upper surface ofthe main body into the main body. The second portion is extended alongthe upper surface of the main body. The first well is located in themain body and below the second portion. The first well is kept away fromthe first portion with a predetermined distance. The source region islocated in the first well. The second well is located in the main bodyand extends from a bottom of the first portion to a place close to adrain region.

In an embodiment of the present invention, the first portion isconnected to the second portion and the second portion is electricallyconnected to a gate electrode.

In an embodiment of the present invention, the first portion and thesecond portion are separated by a dielectric layer. The first portion iselectrically connected to a gate electrode, whereas the second portionis electrically connected to the source region.

A fabrication method of a high-voltage metal oxide semiconductor deviceis also provided in the present invention. The fabrication methodcomprises the steps of: (a) providing a substrate; (b) forming a firstepitaxial layer of a first conductivity type on the substrate; (c)defining a doping region in the first epitaxial layer by using a maskand implanting dopants of a second conductivity type in the firstepitaxial layer to form a first doped region; (d) repeating steps (b)and (c) more than once; (e) forming a second epitaxial layer of thefirst conductivity type on the first epitaxial layers; (f) forming atrench exposing the uppermost first doped region; (g) forming aconductive structure with a first portion and a second portion on thesecond epitaxial layer, the first portion located in the trench, and thesecond portion extended along an upper surface of the second epitaxiallayer; (h) implanting dopants of the second conductivity type in thesecond epitaxial layer by using the conductive structure as a mask toform a plurality of first wells, which is away from the first portionwith a predetermined distance; and (i) forming a plurality of sourceregions of the first conductivity type in the first wells.

Another high-voltage metal oxide semiconductor device is provided in thepresent invention. The high-voltage metal oxide semiconductor devicecomprises a main body of a first conductivity type, a gate conductivelayer, two first wells of a second conductivity type, two source regionsof the first conductivity type, and a second well of the secondconductivity type. The gate conductive layer extends along an uppersurface of the main body. The two first wells are located in the mainbody and corresponding to the two opposite edges of the gate conductivelayer respectively. The two source regions are located in the two firstwells and beneath the two opposite edges of the gate conductive layerrespectively. The second well is located in the main body and extendedbeneath the gate conductive layer to a place close to a substrate. Thesecond well should be electrically connected to a gate electrode or asource electrode. The second well is away from the two first wells witha predetermined distance. In addition, the distance between the secondwell and the gate conductive layer is greater than depth of the firstwell.

Another fabrication method of a high-voltage metal oxide semiconductordevice is provided in the present invention. The fabrication methodcomprises the steps of: (a) providing a substrate; (b) forming a firstepitaxial layer of a first conductivity type on the substrate; (c)defining a doping region in the first epitaxial layer by using a maskand implanting dopants of a second conductivity type in the firstepitaxial layer to form a first doped region; (d) repeating steps (b)and (c) more than once; (e) forming a second epitaxial layer of thefirst conductivity type on the first epitaxial layers, wherein the firstdoped regions are expanded by heat and mutually connected to form avertical well; (f) forming a guard ring of the second conductivity typein the second epitaxial layer to define an active region, and the guardring being planarly overlapped with the vertical well; (g) forming agate conductive layer on an upper surface of the second epitaxial layerand aligned to the vertical well; (h) implanting dopants of the secondconductivity type in the second epitaxial layer by using the gateconductive layer as a mask and driving in the dopants to form aplurality of first wells away from the vertical well with apredetermined distance, meanwhile, the guard ring being expandeddownward to connect to the vertical well; and (i) forming a plurality ofsource regions of the first conductivity type in the first wells

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be specified with reference to itspreferred embodiment illustrated in the drawings, in which:

FIGS. 1 and 1A show the contribution of different portions in thetraditional high-voltage semiconductor field effect transistor to theoverall on-resistance under different withstanding voltage;

FIGS. 2A and 2B are schematic cross-section view showing a preferredembodiment of a high-voltage metal oxide semiconductor device inaccordance with the present invention;

FIG. 3 is a schematic cross-section view showing another preferredembodiment of the high-voltage metal oxide semiconductor device inaccordance with the present invention;

FIGS. 4A to 4H are schematic views showing a preferred embodiment of thefabrication method of a high-voltage metal oxide semiconductor device inaccordance with the present invention;

FIG. 5 is a schematic view of another preferred embodiment of thehigh-voltage metal oxide semiconductor in accordance with the presentinvention;

FIGS. 6A to 6E are schematic views showing another preferred embodimentof the fabrication method of a high-voltage metal oxide semiconductordevice in accordance with the present invention;

FIG. 7 is a schematic cross-section view showing preferred embodiment ofthe first portion of the conductive structure of FIG. 6E beingelectrically connected to the source region in accordance with thepresent invention;

FIG. 8 is a schematic cross-section view showing still another preferredembodiment of the high-voltage metal oxide semiconductor device inaccordance with the present invention;

FIG. 9A is a schematic cross-section view showing a preferred embodimentof the second well of FIG. 8 being electrically connected to the sourceelectrode in accordance with the present invention;

FIG. 9B is a schematic cross-section view showing a preferred embodimentof the second well of FIG. 8 being electrically connected to the gateelectrode in accordance with the present invention; and

FIGS. 10A to 10C are schematic views showing a preferred embodiment ofthe fabrication method of the high-voltage metal oxide semiconductordevice of FIG. 8 and the guard ring thereof in accordance with thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIGS. 2A and 2B are schematic views showing a preferred embodiment of ahigh-voltage metal oxide semiconductor device in accordance with thepresent invention. An N-type metal oxide semiconductor device fieldeffect transistor (MOSFET) is described below as an example. As shown,the high-voltage metal oxide semiconductor device has an N-typeepitaxial layer 120, a conductive structure 150, a P-type first well160, an N-type source region 170, and a P-type second well 130. TheN-type epitaxial layer 120 is located on an N-type substrate 110 as themain body of the high-voltage metal oxide semiconductor device. Theconductive structure 150 is located on the N-type epitaxial layer 120.The conductive structure 150 showing a T-shaped cross-section has afirst portion 152 and a second portion 154. The first portion 152 isextended from an upper surface of the N-type epitaxial layer 120 intothe N-type epitaxial layer 120. The second portion 154 is extended alongthe upper surface of the N-type epitaxial layer 120. The conductivestructure 150 is electrically connected to a gate electrode G.

The P-type first well 160 is located in the N-type epitaxial layer 120below the second portion 154 of the conductive structure 150. Inaddition, the first well 160 is kept away from the first portion 152 ofthe conductive structure 150 with a predetermined distance. That is,there is an N-type area interposed between the P-type first well 160 andthe first portion 152 of the conductive structure 150. The N-type sourceregion 170 is located in the P-type first well 160 and is correspondingto the edge of the second portion 154 of the conductive structure 150.The N-type source region 170 and the N-type epitaxial layer 120 areseparated by the P-type first well 160. The source region 170 iselectrically connected to a source electrode S.

The P-type second well 130 is located in the epitaxial layer 120 and isextended from a bottom of the first portion 152 downward to a placeclose to the N-type substrate 110. The bottom of the P-type second well130 and the N-type substrate 110 are separated by N-type area withenough thickness. In addition, the P-type second well 130 is not incontact with the first portion 152 of the conductive structure 150. TheP-type second well 130 is separated from the first portion 152 by atleast one oxide layer 140. However, the P-type second well 130 can't befar away from the first portion 152 of the conductive structure 150. Inorder to ensure that the charges in the second well 130 can be inducedby the potential of the first portion 152, the P-type second well 130should be adjacent to the first portion 152. Moreover, there should bean N-type area with enough thickness left between the P-type second well130 and the P-type first well 160 as a conduction path when the metaloxide semiconductor device is conducting.

Referring to FIG. 2A, as the voltage difference (V_(GS)) between thegate electrode G and the source electrode S of the metal oxidesemiconductor device is smaller than a threshold voltage (V_(TH)), thechannel connecting the source region 170 and the N-type epitaxial layer120 does not exist in the P-type first well 160. Meanwhile, when aforward bias is applied between the drain electrode D and the sourceelectrode S, the range of the depletion region on the interface betweenthe P-type first well 160, which is electrically connected to the sourceelectrode S, and the N-type epitaxial layer 120, which is electricallyconnected to the drain D, is expanded as the dashed line shows.

It is noted that the voltage levels of the gate electrode G and thesource electrode S are substantially the same when the metal oxidesemiconductor device is turned off. The forward bias applied between thedrain electrode D and the source electrode S also expands the depletionregion on the interface between the P-type second well 130, which isadjacent to the conductive structure 150 to access the potential of thegate electrode G, and the N-type epitaxial layer 120, which iselectrically connected to the drain electrode D, as the dashed lineshows. The depletion regions formed on the interfaces between the firstwell 160 and the epitaxial layer 120 and between the second well 130 andthe epitaxial layer 120 may block the conduction path from the N-typesubstrate 110 to the source region 170. Because the depletion regionshows perfect voltage blocking capability, the withstanding voltage ofthe metal oxide semiconductor device is thus enhanced dramatically.

Referring to FIG. 2B, when the voltage difference (V_(GS)) between thegate electrode G and the source electrode S is greater than a thresholdvoltage (V_(TH)), a channel is formed in the P-type first well 160 underthe second portion 154 to connect the N-type source region 170 and theN-type epitaxial layer 120. At this time, free electrons in the sourceregion 170 are injected into the depletion region through the channel torecover the conductivity of the N-type epitaxial layer 120 to form theconduction path extended from the source region 170, through the channelunder the second portion 154, turned downward by the side of the firstportion 152 and the second well 130 to the N-type substrate 110.

As shown, for a preferred embodiment, the width of the second well 130is greater than the width of the first portion 152 to restrict thethickness of N-type area interposed between the first well 160 and thesecond well 130 so as to control the time needed to retrieve theconductivity of the N-type epitaxial layer 120 when the metal oxidesemiconductor device is turned on. In addition, the upper edge of thesecond well 130 covers the bottom of the first portion of the conductivestructure 150 to ensure that the charges in the second well 130 canquickly respond the potential of the first portion 152. Moreover,withstanding voltage of the high-voltage metal oxide semiconductordevice in accordance with the present invention is positively dependentto the extending distance of the second well 130. The extending distanceof the second well 130 should be much greater than the length of thefirst portion 152 in practice.

The above mentioned embodiment describes a high-voltage metal oxidesemiconductor field effect transistor. However, the scope of the presentinvention should not be limited thereto. For example, if the N-typesubstrate 110 in the above mentioned embodiment is replaced by a P-typesubstrate, an insulated gate bipolar transistor (IGBT) in accordancewith the present invention is shown.

FIG. 3 is a schematic view showing another preferred embodiment of thehigh-voltage metal oxide semiconductor device in accordance with thepresent invention. In contrast with the embodiment as shown in FIGS. 2Aand 2B, the conductive structure 150′ in the present embodiment has andielectric layer 156, such as an oxide layer, interposed between thefirst portion 152′ and the second portion 154′ so as to electricallyisolate the first portion 152′ and the second portion 154′. Moreover, inthe present embodiment, the second portion 154′ of the conductivestructure 150′ is electrically connected to the gate electrode G,whereas the first portion 152′ of the conductive structure 150′ iselectrically connected to the source electrode S.

Referring to FIGS. 2A and 2B, potential of the second well 130 of thehigh-voltage metal oxide semiconductor device is decided by thepotential of the gate electrode G. In contrast, potential of the secondwell 130 of the present embodiment is decided by the potential of thesource electrode S. When the high-voltage metal oxide semiconductordevice is turned off and the voltage difference (V_(GS)) between thegate electrode G and the source electrode S is smaller than thethreshold voltage (V_(TH)), there are depletion regions formed on theinterfaces between the P-type first well 160 and the N-type epitaxiallayer 120 and between the P-type second well 130 and the N-typeepitaxial layer 120 to block the conduction path from the N-typesubstrate 110 to the source region 170 and provide perfect voltageblocking capability.

FIGS. 4A to 4H are schematic views showing a preferred embodiment of thefabrication method of a high-voltage metal oxide semiconductor device inaccordance with the present invention. The fabrication method of anN-type metal oxide semiconductor device is shown as an example.Referring to FIG. 4A, firstly, an N-type substrate 210 is provided.Then, as shown in FIG. 4B, an N-type first epitaxial layer 220 a isformed on the substrate 210. Afterward, a photoresist pattern layer PRis formed on the first epitaxial layer 220 a by using a mask to define adoping region in the first epitaxial layer 220 a. P-type dopants arethen implanted in the first epitaxial layer 220 a through thephotoresist pattern layer PR to form a P-type first doped region 230 a.

Afterward, as shown in FIG. 4C, the fabrication steps of FIG. 4B,including the steps of forming the N-type first epitaxial layer 220 aand forming the P-type first doped region 230 a, are repeated more thanonce. The number of repeated cycles is positive correlated to thewithstanding voltage of the high-voltage metal oxide semiconductor. Forexample, as the metal oxide semiconductor device is designed towithstand a voltage of 600V, the fabrication steps of FIG. 4B arerepeated six times to stack six first epitaxial layers 200 a on thesubstrate 210 with six first doped regions 230 a formed therein.

The fabrication steps of FIG. 4B needs a mask to define the dopingregion in the first epitaxial layer 220 a. In the present embodiment,the repeated fabrication steps use the same mask for defining the firstdoped regions 230 a in each of the first epitaxial layers 220 a so thatsize of the first doped regions 230 a are substantially the same and thefirst doped regions are vertically aligned. In addition, because theepitaxial layer is grown at a high temperature, the range of the firstdoped regions 230 a would be expanded by heat during the step of growingepitaxial layers. In the present embodiment, as shown in FIG. 4C, byproperly controlling the depth and concentration of the implanteddopants as well as the thickness of the respective first epitaxial layer220 a, the first doped regions 230 a in the first epitaxial layers 220 aare able to connect with each other to form a single P-type verticalwell 230 (corresponding to the second well 130 in FIGS. 2A and 2B).However, a predetermined distance must be kept between the P-typevertical well 230 and the N-type substrate 210.

Afterward, referring to FIG. 4D, an N-type second epitaxial layer 220 bis formed on the stacked first epitaxial layers 220 a. The secondepitaxial layer 220 b and the stacked first epitaxial layers 220 acompose an epitaxial layer 220 as the main body of the metal oxidesemiconductor device. Thereafter, a trench 248 is formed in theepitaxial layer 220 to expose the uppermost first doped region 230 a (orthe upper edge of the P-type vertical well 230). Afterward, referring toFIG. 4E, an oxide layer 240 is formed on the exposed surface of thesecond epitaxial layer 220 b. Then, a conductive layer, such as apolysilicon layer (not shown), is deposited on the second epitaxiallayer 220 b and fills the trench 248. After using a mask to define theconductive structure 250 in the polysilicon layer and removing theunwanted polysilicon material by etching, the conductive structure 250has a first portion 252 and a second portion 254 is formed on the secondepitaxial layer 220 b. As shown, the first portion 252 is located in thetrench 248 and the second portion 254 is extended along the uppersurface of the epitaxial layer 220 b.

Afterward, referring to FIG. 4F, P-type dopants are implanted in thesecond epitaxial layer 200 b by using the conductive structure 250 as amask so that a plurality of P-type first wells 260 away from the firstportion 252 with a predetermined distance is formed in the epitaxiallayer 220. That is, there is an N-type area located between the firstwell 260 and the first portion 252. In addition, there is also an N-typearea with enough width located between the P-type first well 260 and theP-type vertical well 230 acting as the conduction path when the metaloxide semiconductor is turned on.

Afterward, referring to FIG. 4G, a photoresist pattern layer PR isformed on the first wells 260 by using a source mask (not shown) todefine a plurality of the source regions 270. Then, N-type dopants areimplanted through the photoresist pattern layer PR to form the sourceregions 270 in the first wells 260, respectively, Thereafter, as shownin FIG. 4H, a dielectric layer 280 is deposited over the exposedsurface. Then, a plurality of contact windows 282 are formed in thedielectric layer 280 to expose the source regions 270 and the firstwells 260 under the dielectric layer 280. Afterward, P-type dopants areimplanted in the first wells 260 by using the dielectric layer 280 as amask to form a plurality of P-type heavily doped regions 290 in thefirst wells 260 respectively.

Referring to FIG. 4H, in the above mentioned embodiment, the first dopedregions 230 a in the epitaxial layer 220 are connected with each otherto form a vertical well 230. However, it should not be used to restrictthe scope of the present invention. For example, referring to FIG. 5,each of the first doped regions 330 a in the epitaxial layer 220 may beseparated with each other under the limitation that the spacing betweenneighboring first doped regions 330 a should be small enough to ensurethat the charges of each of the first doped regions 330 a can be inducedby the first portion 252 of the conductive structure 250.

FIGS. 6A to 6E are schematic views showing another preferred embodimentof a high-voltage metal oxide semiconductor device in accordance withthe present invention. Following the step of FIG. 4D, referring to FIG.6A, a first oxide layer 241 is then formed on the exposed surface of thesecond epitaxial layer 220 b. Afterward, a first polysilicon layer isdeposited on the second epitaxial layer 220 b as a whole and fills thetrench 248. Part of the first polysilicon layer is then removed byetching back and only the polysilicon material in the trench 248 is leftto construct the first portion 352 of the conductive structure 350.

Afterward, referring to FIG. 6B, a second oxide layer 242 is formed onthe exposed surface of the first portion 352. Then, a second polysiliconlayer is deposited as a whole to cover the second oxide layer 242.Thereafter, the second portion 354 of the conductive structure 350 isdefined by using a mask (not shown) and the unwanted polysiliconmaterial is removed so as to form the second portion 354 of theconductive structure 350.

Afterward, referring to FIG. 6C, P-type dopants are implanted in thesecond epitaxial layer 220 b directly by using the second portion 354 ofthe conductive structure 350 as a mask so as to form a plurality offirst wells 260. Thereafter, referring to FIG. 6D, a photoresist patternlayer PR is formed on the first wells 260 to define a plurality ofsource regions 270. Then, N-type dopants are implanted in the firstwells 260 to form the source regions in the first wells respectively.Thereafter, as shown in FIG. 6E, a dielectric layer 280 is depositedover the exposed surface. Then, a plurality of contact windows 282 areformed in the dielectric layer 280 to expose the source regions 270 andthe first wells 260 under the dielectric layer 280. Afterward, P-typedopants are implanted in the first wells 260 by using the dielectriclayer 280 as a mask to form a plurality of P-type heavily doped regions290 in the first wells 260 respectively.

Referring to FIGS. 6A and 6B, the first portion 352 and the secondportion 354 of the conductive structure 350 are separated by the secondoxide layer 242. The second portion 354 is electrically connected to thegate electrode G to control the switching of the metal oxidesemiconductor device, whereas the first portion 352 may be electricallyconnected to the source electrode S. Referring to FIG. 7, in order toelectrically connect the first portion 352 and the source electrode S,as a preferred embodiment, there is an opening 284 formed in thedielectric layer 280 adjacent to the edge of the high-voltage metaloxide semiconductor device to expose the first portion 352 and a sourcemetal layer 295 connecting the source regions 270 is tilled into theopening 284 to connect the first portion 352.

FIG. 8 is a schematic view showing another preferred embodiment of thehigh-voltage metal oxide semiconductor device in accordance with thepresent invention. An N-type high-voltage metal oxide semiconductorfield effect transistor is described below as an example. As shown, thehigh-voltage metal oxide semiconductor device has an N-type epitaxiallayer 120, a gate conductive layer 450, two P-type first wells 160, twoN-type source regions 170, and a P-type second well 130. The N-typeepitaxial layer 120 is located on an N-type substrate 110 as a main bodyof the high-voltage metal oxide semiconductor device. The gateconductive layer 450 is extended along the upper surface of the N-typeepitaxial layer 120. The two first wells 160 are located in the N-typeepitaxial layer 120 and corresponding to the two opposite edges of thegate conductive layer 450 respectively. In addition, the two first wells160 are spaced apart from each other.

The two source regions 170 are located in the two first wells 160 andbeneath the two opposite edges of the gate conductive layer 450,respectively. The P-type second well 130 is located in the N-typeepitaxial layer 120 and extended beneath the gate conductive layer 450to a place close to the N-type substrate 110, which may be regarded asan N-type drain region. The P-type second well 130 is away from the twoP-type first wells 160 with a predetermined distance and is electricallyconnected to the gate electrode G or the source electrode S. As apreferred embodiment, the distance between the second well 130 and thegate conductive layer 450 should be greater than the depth of the firstwell 160.

Also referring to FIGS. 9A and 9B, in order to electrically connect thesecond well to the gate electrode G or the source electrode S of thehigh-voltage metal oxide semiconductor device, the guard ring 460located near the edge of the high-voltage metal oxide semiconductordevice may be used as an interconnection structure. As shown, the P-typeguard ring 460 is formed in the N-type epitaxial layer 120 andsurrounding the P-type first well 160 in the active region A. The depthof the guard ring 460 is greater than the depth of the P-type first well160. The P-type second well 130 is extended from the active region A tothe edge of the high-voltage metal oxide semiconductor device andconnected to the guard ring 460.

Referring to FIG. 9A, an opening 186 is formed in the dielectric layer180 to expose the guard ring 460 and a source metal layer 195 depositedon the dielectric layer is connected to the source region 170 and theguard ring 460. Thereby, the P-type second well 130 is electricallyconnected to the source electrode S through the guard ring 460 and thesource metal layer 195. Referring to FIG. 9B, the gate conductive layer450′ near the edge of the active region A is extended toward the uppersurface of the guard ring 460 and connected to the guard ring 460.Thereby, the P-type second well 130 is electrically connected to thegate electrode G through the guard ring 460 and the gate conductivelayer 450′.

FIGS. 10A to 10C are schematic views showing a preferred embodiment ofthe fabrication method of the metal oxide semiconductor device as shownin FIGS. 8 with the guard ring 460. Following the step of FIG. 4C,referring to FIG. 10A, the second epitaxial layer 220 b is then formedon the stacked first epitaxial layers 220 a and a P-type guard ring 460is formed in the second epitaxial layer 220 b to define an active regionA. Turns to the top view, the guard ring 460 would be planarlyoverlapped with the P-type vertical well 230 (corresponding to thesecond well 130 of FIG. 8) in the epitaxial layer 220. Then, a gateconductive layer 450 is formed on the upper surface of the secondepitaxial layer 220 b and vertically aligned to the vertical well 230.

Afterward, referring to FIG. 10B, P-type dopants are implanted in thesecond epitaxial layer 220 b by using the gate conductive layer 450 as amask. The implanted P-type dopants are then driven into the secondepitaxial layer 220 b by heat to form a plurality of P-type first wells260 away from the vertical well 230 with a predetermined distance. It isnoted that the above mentioned step of driving-in the P-type dopantsalso drives the P-type dopants of the guard ring 460 into the secondepitaxial layer 220 b so as to expand the range of the guard ring 460downward to connect with the P-type vertical well 230.

Afterward, referring to FIG. 10C, the location of a plurality of sourceregions 270 is defined by using a source mask (not shown), and thenN-type dopants are implanted in the first wells 260 to form the sourceregions 270. Thereafter, a dielectric layer 280 is deposited over theexposed surface. Then, a plurality of contact windows 282 are formed inthe dielectric layer 280 to expose the source regions 270 and the firstwells 260 under the dielectric layer 280. Afterward, P-type dopants areimplanted in the first wells 260 through the dielectric layer 280 toform a plurality of P-type heavily doped regions 290 in the first wells260.

The high-voltage metal oxide semiconductor device provided in thepresent invention has the following advantages:

Firstly, referring to FIGS. 2A and 2B, as the voltage difference(V_(GS)) between the gate electrode G and the source electrode S of themetal oxide semiconductor device is smaller than a threshold voltage(V_(TH)), the forward bias applied between the drain electrode D and thesource electrode S may result in the expansion of depletion regions toblock the N-type area between the first well 160 and the second well130. The depletion region shows perfect voltage blocking capability.Therefore, the withstanding voltage of the metal oxide semiconductordevice can be dramatically increased. On the other hand, as the voltagedifference (V_(GS)) between the gate electrode G and the sourceelectrode S of the metal oxide semiconductor device is greater than athreshold voltage (V_(TH)), a channel is formed in the P-type first well160 between the N-type source region 170 and the N-type epitaxial layer120. Free electrons of the source region 170 are injected to thedepletion region through the channel to recover the conductivity of theN-type area to rebuild the conduction path from the source region 170 tothe substrate 110. It is noted that the on-resistance of the metal oxidesemiconductor device is related to the dope concentration of the N-typeepitaxial layer 120 but the withstanding voltage is not. Therefore, itis possible to achieve the object of low on-resistance by increasingdope concentration but remain high voltage blocking capability at thesame time.

Secondly, referring to FIG. 2A, the depletion region for blocking theconduction path is formed between the first well 160 and the second well130. The distance between the first well 160 and the second well 130 issmaller than the distance between two neighboring first wells 160.Therefore, in contrast with the traditional high-voltage metal oxidesemiconductor devices with lateral PN junctions, such as Coolmos™ andsuper junction device, which need to recover the conductivity of thedepletion region with a width substantially identical to the distancebetween neighboring first wells 160 to rebuild the conduction path, thetime needed to rebuild the conduction path in accordance with thehigh-voltage metal oxide semiconductor device in accordance with thepresent invention is much faster.

In addition, referring to FIG. 2A, there are two intrinsic zener diodes,one is located between the heavily doped region 190, the first well 160,and the N-type epitaxial layer 120, and the other is located between thesecond well 130 and the N-type epitaxial layer 120. When avalanchebreakdown happens, avalanche current would be shared by the two zenerdiodes rather than concentrated to the zener diode between the heavilydoped region 190, the first well 160, and the N-type epitaxial layer120. Therefore, the current flowing through the resistor traversing theepitaxial layer below the second portion 154 is reduced so as to preventthe bipolar junction transistor between the N-type epitaxial layer 120,P-type first well 160, and the source region 170 from being damaged bylarge current.

While the preferred embodiments of the present invention have been setforth for the purpose of disclosure, modifications of the disclosedembodiments of the present invention as well as other embodimentsthereof may occur to those skilled in the art. Accordingly, the appendedclaims are intended to cover all embodiments which do not depart fromthe spirit and scope of the present invention.

1. A high-voltage metal oxide semiconductor device comprising: a mainbody of a first conductivity type; a gate conductive layer, extendedalong an upper surface of the main body; two first wells of a secondconductivity type, located in the main body corresponding to twoopposite edges of the gate conductive layer; two source regions of thefirst conductivity type, located in the two first wells respectively andbeneath the two opposite edges of the gate conductive layer; a secondwell of the second conductivity type, which is located in the main bodyand extended beneath the gate conductive layer to a place close to asubstrate, being electrically connected to a gate electrode or a sourceelectrode and away from the two first wells with a predetermineddistance, and a distance between the second well and the gate conductivelayer being greater than depth of the first well; and a guard ring ofthe second conductivity type, located in the main body and surroundingthe first wells, a depth of the guard ring being greater than that ofthe depth of the first well, and a lower edge of the guard ring beingconnected to the second well.
 2. The high-voltage metal oxidesemiconductor device as claimed in claim 1, wherein the guard ring iselectrically connected to the source electrode through a source metallayer.
 3. The high-voltage metal oxide semiconductor device as claimedin claim 1, wherein the guard ring is electrically connected to the gateelectrode through the gate conductive layer.